The present invention relates to an improved internal step-down converter, which may be built in a semiconductor integrated circuit for stepping an external supply voltage down to a predetermined voltage.
A conventional internal step-down converter includes a current-mirror differential amplifier 1 and a driver 2 as shown in FIG. 8. The differential amplifier 1 generates a differentially amplified output voltage VDRV by amplifying a potential difference between a reference voltage VREF and an internally-stepped-down supply voltage VINT. The driver 2 may be implemented as a p-channel MOSFET, which receives the voltage VDRV at its gate and supplies a current in such an amount as to regulate the voltage VINT at a predetermined value.
In the internal step-down converter with such a configuration, as a load current ILOAD increases, the internally-stepped-down supply voltage VINT decreases. On and after the internally-stepped-down supply voltage VINT has decreased to be lower than the reference voltage VREF, the differentially amplified output voltage VDRV of the differential amplifier 1 becomes low. In such a situation, the current-handling capability of the driver 2 is enhanced to raise the internally-stepped-down supply voltage VINT. On the other hand, once the internally-stepped-down supply voltage VINT exceeds the reference voltage VREF, the differentially amplified output voltage VDRV becomes high. As a result, the current-handling capability of the driver 2 declines or the driver 2 stops supplying the current. In this manner, the internally-stepped-down supply voltage VINT is regulated at the reference voltage VREF.
In the conventional internal step-down converter, however, the output voltage of the differential amplifier 1 has an amplitude smaller than a potential difference .vertline.VDD-VSS.vertline. between external supply voltages VDD and VSS. Thus, the current-handling capability of the driver 2 cannot be made full use of. Accordingly, to increase the load-current-handling capability of the driver 2, the p-channel MOSFET, which constitutes the driver 2, should have its channel width increased. Also, to maintain the transient response speed of the internal step-down converter, a constant current IS, always flowing through the differential amplifier 1, should be increased. That is to say, to increase the load-current-handling capability of the internal step-down converter, the layout area and current consumed should be increased, thus interfering with the downsizing of, and reduction in power consumed by, a semiconductor integrated circuit.
Furthermore, if the external supply voltage VDD is set low, then the output voltage VDRV of the differential amplifier 1 has its amplitude decreased. As a result, the current-handling capability of the driver 2 declines drastically. Accordingly, it is very difficult to supply a constant internally-stepped-down supply voltage VINT to the semiconductor integrated circuit.